1. Field of the Invention
The present invention relates to a facsimile communication system, more particularly to a communication apparatus that suppresses proprietary protocols in order to assure successful facsimile communication.
2. Description of the Related Art
Voice communication apparatus that relays voice packets on a network such as the Internet commonly accommodates facsimile terminals as well as telephone terminals. Facsimile data have traditionally been sent over voice channels in telephone networks by procedures specified in Recommendation T.30 (‘Procedures for Document Facsimile Transmission in the General Switched Telephone Network’) of the Telecommunications Sector of the International Telephone and Telegraph Union (ITU-T). For transmission over the networks such as the Internet, however, the Internet Facsimile Protocol (IFP) described in ITU-T Recommendation T.38 (‘Procedures for Real-Time Group-3 Facsimile Communication over IP Networks’) is frequently used. The T.38 facsimile data transmission protocol is based on the T.30 protocol, but T.38 call connection procedures are based on a protocol such as the one given in ITU-T Recommendation H.323 (‘Packet Based Multimedia Communication Systems’). Details can be found in Japanese Patent Application Publications No. 2006-5482, 2004-32007, and 2003-244383.
ITU-T Recommendation T.38 is usually used on networks of limited bandwidth that carry voice data in a compressed form, as specified in Annex A of ITU-T Recommendation G729 (‘Coding of Speech at 8 kb/s Using Conjugate-Structure Algebraic-Code-Excited Linear Prediction’), for example. The voice transmission equipment in these networks compresses voice data at the transmitting end and decompresses the data at the receiving end, but the compression and decompression process is lossy; the decompressed data generally differ somewhat from the original data.
For voice data, minor data alterations can easily be tolerated, but in a facsimile transmission, such alterations tend to cause noticeable defects in the received facsimile document. For this reason, voice transmission equipment that compresses voice data conventionally transmits the facsimile data as signaling data instead of voice data, converting the facsimile data to IFP packets and transmitting the IFP packets through a control signal communication channel.
When facsimile signals received from a facsimile terminal are converted to IFP packets for transmission according to ITU-T Recommendation T.38, the facsimile data are analyzed and restrictions such as the following are imposed.
(1) Facsimile communication according to ITU-T Recommendation V.34 (‘A Modem Operating at Data Signaling Rates of up to 33600 bps for Use on the General Switched Telephone Network and on Lease Point-to-Point, Two-Wire, Telephone-Type Circuits’) is blocked, because ITU-T Recommendation T.38 only supports facsimile transmission up to ITU-T Recommendation V.17 (‘A 2-Wire Modem for Facsimile Applications with Rates up to 14400 bits/s’).
(2) If the manufacturer of the facsimile terminal equipment has used the non-standard facilities (NSF) signal provided in ITU-T Recommendation T.30 to implement proprietary communication functions (communication functions other than those specified in ITU-T Recommendation T.30), the NSF signal will be blocked by voice communication equipment that carries out facsimile transmission according to ITU-T Recommendation T.38, because the voice communication equipment cannot handle the manufacturer's proprietary communication system. That is, any facsimile data that are determined to represent an NSF signal will be discarded instead of being transmitted to the destination facsimile terminal.
Facsimile transmission through conventional voice communication equipment complying with ITU-T Recommendation T.38 will be further described with reference to FIGS. 1 to 3.
FIG. 1 illustrates facsimile transmission from a facsimile terminal 1A to a facsimile terminal 1B. Facsimile terminal 1A is accommodated by communication apparatus 100A, which transmits to communication apparatus 100B, which accommodates facsimile terminal 1B. Facsimile terminal 1B has proprietary functions that cause it to send NSF signals. Communication apparatus 100A comprises a digital signal processing unit 110A, a jitter absorption buffer 120A, a voice processing unit 130A, a facsimile communication processing unit 140A, and a call processing unit 150a. Communication apparatus 100B comprises similar components with similar reference characters, the final A being changed to B.
FIG. 2 illustrates a typical communication signaling sequence. To initiate facsimile communication, facsimile terminal 1A originates a call to the receiving facsimile terminal and sends a calling tone (CNG) as specified in ITU-T Recommendation T.30 (S510-1).
The digital signal processing unit 110A in communication apparatus 100A detects the calling tone sent from facsimile terminal 1A and notifies the facsimile communication processing unit 140A (S510-2), which converts the calling tone to an IFP packet. This CNG packet is sent on a control signal channel P2 on the network (NW) and received by the facsimile communication processing unit 140B of communication apparatus 100B (S510-3). Facsimile communication processing unit 140B notifies the digital signal processing unit 110B in communication apparatus 100B (S510-4), which sends a calling tone to facsimile terminal 1B (S510-5).
When facsimile terminal 1B receives the calling tone, it sends a called station identification tone (CED) toward facsimile terminal 1A (S520-1) as specified in ITU-T Recommendation T.30.
The CED tone is detected by the digital signal processing unit 110B in communication apparatus 100B, the facsimile communication processing unit 140B is notified (S520-2), and the tone is converted to an IFP packet which is sent over the control signaling channel P2 on the network to communication apparatus 100A (S520-3). There the IFP packet is received by the facsimile communication processing unit 140A, the digital signal processing unit 110A is notified (S520-4), and digital signal processing unit 110A sends a CED tone to facsimile terminal 1A (S520-5).
Next, the receiving facsimile terminal 1B sends a preamble signal to the transmitting facsimile terminal 1A as specified in ITU-T Recommendation V.21 (‘300 Bits per Second Duplex Modem Standardized for Use in the General Switched Telephone Network’). The preamble signal is transmitted (S530-1 to S530-5) in the same way as the CED tone signal (S520-1 to S520-5), being converted to IFP packet form for transmission between the communication apparatuses 100A and The receiving facsimile terminal 1B continues by sending an NSF signal toward the transmitting facsimile terminal 1A (S540-1, S540-2). The digital signal processing unit 110B in communication apparatus 100B receives and analyzes the NSF signal, finds that it relates to a proprietary function introduced by the manufacturer of facsimile terminal 1B, and discards the NSF signal (S550) without passing it on. The communication apparatuses detect facsimile signals at fixed intervals of, for example, one hundred milliseconds (100 ms), so a single NSF signal may sometimes be received as two separate signals, as illustrated.
When the receiving facsimile terminal 1B has finished sending its NSF signal, it sends a Called Subscriber Identification (CSI) signal and a Digital Information Signal (DIS) to the transmitting facsimile terminal 1A as specified in ITU-T Recommendation T.30 (S560-1 to S560-5 and S570-1 to S570-5). The sequence continues with further signals, which will not be described.
As illustrated by this sequence, when the receiving facsimile terminal 1B sends an NSF signal, the NSF signal is blocked at the communication apparatus 100B to which the receiving facsimile terminal 1B is connected, while the signals following the NSF signal are transmitted onward as if the NSF signal had not be sent at all. This enables the communication apparatus 100B to carry out facsimile transmission according to ITU-T Recommendation T.38.
The transmitting facsimile terminal 1A measures the elapsed time from reception of the preamble signal from facsimile terminal 1B to reception of the following CSI or DIS signal. The value Ta of this elapsed time is:
                    Ta        =                              preamble            ⁢                                                  ⁢            signal            ⁢                                                  ⁢            transmission            ⁢                                                  ⁢            time                    +                      NSF            ⁢                                                  ⁢            signal            ⁢                                                  ⁢            transmission            ⁢                                                  ⁢            time                                              (        1        )            If the transmitted facsimile data are buffered for a certain time in order to absorb network jitter, the value of Ta is increased accordingly:
                    Ta        =                              preamble            ⁢                                                  ⁢            signal            ⁢                                                  ⁢            transmission            ⁢                                                  ⁢            time                    +                      NSF            ⁢                                                  ⁢            signal            ⁢                                                  ⁢            transmission            ⁢                                                  ⁢            time                    +                      buffering            ⁢                                                  ⁢            time            ⁢                                                  ⁢            to            ⁢                                                  ⁢            absorb            ⁢                                                  ⁢            network            ⁢                                                  ⁢            jitter                                              (        2        )            
A problem with the practice of blocking NSF signals at the communication apparatus is that the functions implemented by facsimile manufacturers by use of the NSF signal have become increasingly complex. This is especially true when the facsimile terminal is a multi-function device that may also operate as a printer, scanner, and copier. The lengths of NSF signals have consequently increased to the point where, in the sequence in FIG. 2, the transmitting facsimile terminal 1A may time out while communication apparatus 100B is blocking the NSF signal, before the CSI or DIS signal can be transmitted, in which case the facsimile terminal 1A disconnects the call and the facsimile transmission fails.
Such a failure is illustrated in FIG. 3. The CNG and CED tones and V.21 preamble are transmitted as in FIG. 2. When the transmitting facsimile terminal 1A receives the V.21 preamble (S530-5), it starts a timer that measures a predetermined time-out time Tb. The facsimile terminal 1B now begins a long period of NSF signal transmission (S540-1 to S540-n) which is blocked (S550) at communication apparatus 100B. After completing the NSF signal transmission, the receiving facsimile terminal 1B sends CSI and DIS signals (S560-1, S570-1), but the time Ta that elapses before the first of these signals reaches the transmitting facsimile terminal 1A (S560-5) exceeds the time-out time Tb. At the point when time Tb has elapsed, facsimile terminal 1A disconnects (S580), so it does not receive the CSI and DIS signals, or receives them but ignores them, and the facsimile transmission fails.